1. Field of the Invention
The field of the invention is that of mobile radio systems, in particular GSM mobile radio systems.
2. Description of the Prior Art
In GSM systems mobiles move around in a network of adjoining cells. Each cell has a separate base transceiver station. Communications involving a mobile in a given cell pass through the base transceiver station of that cell.
Call set-up procedures are governed by a specific protocol with which the present invention is not concerned. When call set-up is completed a specific bidirectional traffic channel (TCH) is assigned to each mobile. The same traffic channel is time-division multiplexed for use by a plurality of mobiles (usually eight at this time). According to the current GSM standard a traffic channel is defined by a list of frequencies. An algorithm controlling hopping between the frequencies in this list can make transmission relatively insensitive to fading.
Allocation of the channels, and consequently of the list of frequencies, in the various cells of the network is managed by a central module of the network. The allocation of frequencies in the network, and where applicable the frequency hopping rules, are organized in such a way as to avoid intracell and intercell interference.
Each traffic channel is allocated a given list of frequencies in a manner that in theory is relatively stable. However, the central module of the network may require to redefine the channel frequencies. This may be to alleviate loss of a carrier, to remove risks of interference caused by expansion of the network, etc.
The conditions under which the channel frequencies are redefined are by no means immaterial. Problems can arise with the continuity of the calls in progress, in two situations in particular:
if a mobile is in the process of changing channel within the same cell around the time the frequencies of the source channel and/or the target channel are redefined; to use the GSM terminology, this can represent either an "immediate assignment" (changing from a broadcast channel to a dedicated channel) or a "normal assignment" (transfer between two dedicated channels); this is described in more detail below; PA1 if a mobile is in the process of changing cell ("hand-over") around the time the frequencies of the origin and/or target channel are redefined. PA1 at least a first time slot is freed by eliminating all calls in said time slot; PA1 frequency redefinition of said first time slot freed is effected by assigning it said second list of M frequencies, after which said time slot is made available for traffic again; PA1 a second time slot is freed by transferring calls from said second time slot to said first time slot; PA1 frequency definition of said second time slot freed is effected by assigning it said second list of M frequencies, after which said second time slot is again made available for traffic; PA1 the transfer of calls from time slot to time slot is continued iteratively until frequency redefinition has been effected for all the time slots of the communication channel. PA1 all new transfers are effected directly to a channel associated with the new list of frequencies; PA1 all calls set up at the time of frequency redefinition are progressively transferred to a channel associated with the new list of frequencies; PA1 for all transfers in progress, i.e. for which an "immediate assignment" has been initiated but not completed, it is necessary to allow for the fact that, on the first access, the mobile takes some time to complete the access procedure: it is then necessary to allocate the channels of the new time slot as soon as it is redefined but to avoid reconfiguring the old time slot until it is certain that the mobile has obtained access after the "immediate assignment" (reception of the GSM signal "ASSIGNMENT COMPLETE"), or that the transfer has been aborted.
The problem also arises of the behavior of a mobile station if this cannot carry out the redefinition of its traffic channel frequencies. This can happen in the event of a protocol error, if the mobile cannot read or interpret the signalling messages relating to the required frequency redefinition.
Various solutions have been proposed to these various problems.
The solution to loss of communication with a mobile because the mobile is unable to execute the frequency redefinition message currently adopted is for the mobile to ignore the lost message. Consequently, the mobile continues to transmit on the old frequencies, until loss of communication is detected. The drawback of this is that during the time which elapses before transmission by the mobile is interrupted this transmission is a potential source of interference.
Two solutions have been proposed for the problem of virtually simultaneous frequency redefinition and channel or cell changing.
A first solution, recommended in standard 04.08.V3.13.0, entails systematically delaying the channel or cell change so that it coincides with frequency redefinition. Assuming a cell change, there will therefore be alignment to the time of frequency redefinition in the target cell. Thus, with this solution, the channel or cell change is effected with the new frequencies.
This first solution has various drawbacks, including some doubt as to what constitutes an acceptable time-delay. If the time-delay is too long there may be significant long-term deterioration of the link, especially in the case of a cell change. This risk is by no means negligible, all the more so in that the GSM system provides a kind of hysteresis to prevent unnecessary switching between cells in the case of a mobile moving along an intercell boundary. The cell change is not effected until the mobile has clearly and finally left the old cell, and is achieved at the cost of some slight and temporary deterioration of communication quality during the time taken to verify that exit from the cell is durable; the proposed switching time-delay would accentuate this deterioration.
In theory it would be possible to impose a maximum time-delay to reduce the risk of call quality deterioration, especially if the network imposes a maximal time-delay between announcement of frequency redefinition and its execution. Apart from the penalty that this represents for the network, this maximal time-delay also has the drawback that it cannot be significantly reduced as it is necessary to retain sufficient time to start up carriers, perform synchronization, etc.
A second solution, currently under assessment, offers better flexibility but is more complex.
The basic principle of this second solution is to have the mobile manage four lists of frequencies simultaneously, namely the old and new frequency definitions for the origin channel and those for the target channel (whether the target channel is in the same cell (assignment) or in a new cell (handover)). On changing cell this principle enables the mobile to revert to the old cell if it does not "lock on" in the new cell. However, this increases commensurately the memory capacity requirement, the on-board processing capacity requirement and the size of the signalling messages to be exchanged.
A third solution described in French patent application No. 93.02308 (now French Patent No. 2,702,109) proposes a middle path between the first solution, which is somewhat unreliable, and the second solution, which is complex to implement both in the mobile and in the network. It involves telling the mobile, at the time of the cell change command, the time at which a frequency change is scheduled in the destination cell. The mobile, knowing also the time in the new cell, the time in the current cell and (where applicable) the time at which a frequency change is scheduled in the current cell, can determine the time remaining for it to complete its transfer into the new cell or to return to its current cell if the attempt to change cell should fail. If this time is greater than the time remaining before the frequency change in one of the two cells, it refuses to carry out the intercell transfer (handover) and advises the network of this, so that it can try again later.
Although this method appears to be a good compromise between the previous two solutions, all these methods have the same drawback: virtually all of the complexity is required in the mobile since the network does not have an accurate knowledge of the time throughout the network of cells. These methods also impose a severe constraint in respect of synchronizing frequency change in the network: the frequency redefinition must be carried out at exactly the same time in a cell, in the infrastructure equipment and in the mobile.
An object of the invention is to alleviate the drawbacks of the existing solutions by providing a method that is reliable, imposes no severe constraints on the network or on the mobile and relies entirely on existing procedures between mobiles and the network, so guaranteeing freedom from compatibility problems which are always likely to be encountered when introducing new methods that can interact with existing methods.